Bandwidth guarantee, limit, and priority interactions

Bandwidth guarantee, limit, and priority interactions

After packet acceptance, the FortiGate unit classifies traffic and may apply traffic policing at additional points during processing. It may also apply QoS techniques, such as prioritization and traffic shaping. Traffic shaping consists of a mixture of traffic policing to enforce bandwidth limits, and priority queue adjustment to assist packets in achieving the guaranteed rate.

If you have configured prioritization, the FortiGate unit prioritizes egressing packets by distributing them among FIFO (first in, first out) queues associated with each possible priority number. Each physical interface has six priority queues. Virtual interfaces do not have their own queues, and instead use the priority queues of the physical interface to which they are bound.

Each physical interface’s six queues are queue 0 to queue 5, where queue 0 is the highest priority queue. However, for the reasons described below, you may observe that your traffic uses only a subset of those six queues. Some traffic may always use a certain queue number. Some queuing may vary by the packet rate or mixture of services. Some queue numbers may be used only by through traffic for which you have configured traffic shaping in the security policy that applies to that traffic session. For example:

  • Administrative access traffic will always use queue 0.
  • Traffic matching security policies without traffic shaping may use queue 0, queue 1, or queue 2. Which queue will be used depends on the priority value you have configured for packets with that ToS (type of service) bit value, if you have configured ToS-based priorities.
  • Traffic matching security policies with traffic shaping may use any queue. Which queue will be used depends on whether the packet rate is currently below the guaranteed bandwidth (queue 0), or above the guaranteed bandwidth. Packets at rates greater than the maximum bandwidth limit are dropped.
  • If the global tos-based-priority is low (3), the priority in a traffic-shaper is medium (2) and a packet flows though a policy that refers to the shaper, the packet will be assigned the priority defined by the shaper, in this case medium (2).

Prioritization and traffic shaping behavior varies by your configuration, the service types and traffic volumes, and by whether the traffic is through traffic, or the traffic originates from or terminates at the FortiGate unit itself.

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